Membranes are a promising area of research for energy-related gas separations such as CO2 removal from flue or fuel gas streams as well as natural gas sweetening. Polymeric membranes are cost effective and widely used for gas separation due to the ease of processing. However, in polymeric membranes there is a tradeoff between improving selectivity and permeability. This tradeoff manifests itself in the Robeson upper bound which establishes upper limit combinations of permeability and selectivity for the best performing membranes. At the upper bound, an increase in permeability is met with a decrease in selectivity. Conversely, inorganic membranes have perm-selectivities that are many times higher than traditional polymeric materials but are not economically feasible for large-scale applications. Most ceramic, glass, and zeolitic membrane materials exhibit costs which are orders of magnitude higher per unit of membrane area when compared to polymeric membranes. Furthermore, inorganic membranes are extremely difficult to fabricate into large, defect-free areas. Low surface area/unit volume, reproducibility, and long term stability of inorganic membranes remains a challenge. A promising route to enhance gas transport properties and fabricate membranes which exceed the Robeson upper bound involves forming composite membranes between polymeric materials and inorganic filler particles to yield mixed matrix membranes (MMM). In theory, the advantages of both the polymer (ease of processing, low cost) and the inorganic material (favorable separation properties) can be realized in a MMM.
MMMs traditionally employ rigid hydrophilic zeolites or carbon molecular sieve particles as the inorganic filler phase. These fillers have surfaces usually not compatible with glassy polymers and this incompatibility of surfaces results in defective polymer/filler interfaces. Metal-Organic frameworks for Separations, J. R. Li, et al. Chem. Rev. 112, 869-932. The different non-ideal structures in MMMs have been characterized as interface voids or sieve-in-a-cage, rigidified polymer layer around the inorganic fillers, and particle pore blockage. In order to surpass the Robeson upper bound, the structure of the MMM has to be defect-free at the polymer/filler interface. This ideal morphology is difficult to achieve due to poor polymer/filler adhesion. Overcoming poor adhesion requires careful selection of a filler and polymer which are likely to interact well. However, this limits the selection of polymers and fillers to only those that are likely to form a defect-free interface.
The metal organic framework (MOF) is a relatively new class of porous materials which show promise as adsorbents and membranes for gas separation applications. MOFs are periodic structures that consist of metal ions or clusters interconnected by organic linking ligands. As a result of their organic/inorganic hybrid structure, MOFs may be made more compatible with polymers and show promise as filler materials in MMMs.
MOF structures are highly tailorable: by varying the linking ligand and/or metal ion, one can introduce new functionality, adjust the pore size, and tune the chemical and gas transport properties to a specific application. Additionally, it is possible to control and optimize the interactions between the polymer and the porous MOF filler particles, thereby improving the mechanical and gas separation properties of the MMMs.
Provided herein is a metal organic frameworks functionalized by addition of a pendant functional group. The resulting functionalized metal organic framework has particular applicability to incorporation in a mixed matrix membrane for use in gas separation. Also provided herein is a mixed matrix membrane comprising the functionalized metal organic framework dispersed throughout a polymer matrix.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.